I'd say that Pokemon is not going to get harder main-storyline wise, although I'd hope for it to. With XY (haven't played ORAS) I felt the game was so easy it was boring. Understandably, the EXP Share buff made it easier for beginners, but if they had made Gym Leaders and the Elite 4 not so underlevelled, it would not have been so easy. I want to see more challenging opponents in the next Generation or so, since those sorts of battles really got my blood pumping
I don't think it will be getting harder since Game Freak will just fit way more Pokemon into a region again at the cost of less new introductions (XY). A larger region would certainly be viable in the future as the capabilities of handhelds grow too (man, a '2 regions in 1 game style' would be pretty epic to see again) and hopefully GF pull it off. It will bring adventure back to the Pokemon games. Right now, although competitive is really fun, the main-story is disappointing and, unlike games of the past Generations where I only played for the storyline and replayed them multiple, I couldn't play the storyline in XY more than twice; it was too tedious.
If they made Pokemon harder to find or catch, that would make it a lot harder to complete the Pokedex again and I would like that, as it would make creating a living dex more of an accomplishment.